LED Calculator. Calculation of Current-Limiting Resistors for a Single LED and LED Arrays![]() This LED calculator designs a simple circuit with one-LED and a series resistor or a multi-LED array circuit with series-LED chains combined in parallel clusters. If you are a beginner in electronics or a university student, you can use it to learn about light-emitting diodes. If you already know how to design LED arrays, you can use it to check your calculations. Example: Calculate a series-parallel array consisting of 30 LEDs for 12 V source voltage, 2 V LED forward voltage, and 20 mA diode forward current. Input Power supply voltage Vs V LED forward current If mA Select LED type or LED forward voltage Vf V Number of LEDs in the multi-LED Array Nt Number of LEDs in a series chain. If not entered, it will be calculated automatically. Ns Output Circuit Diagram Wiring Diagram Resistor nominal and power rating for a series chain with a maximum number of LEDs: Resistor nominal and power rating for an additional series chain: Total power dissipated on all series resistors: Total power dissipated on all LEDs: Total power dissipated by the multi-LED array: Total current from the power supply: Total number of LEDs in the array: Number of series chains connected in parallel: Number of LEDs in the series chain: Number of LEDs in the additional series chain: Definitions and Formulas Used for CalculationSingle LEDA light-emitting diode (LED) is a semiconductor light source with two or more leads. Monochrome LEDs usually have two leads, bi-color may have two or three leads and tri-color and RGB LEDs usually have four leads. An LED emits light when a suitable voltage is applied across its leads. ![]() Conventional infrared LED and its electronic symbol. The square semiconductor die is installed on the negative (cathode) lead. The thin wire connects the square semiconductor die to the positive (anode) lead. To power a single LED, a simple LED circuit with a series current limiting resistor is used. The resistor is necessary because the voltage drop across an LED is approximately constant over a wide range of operating currents.
LEDs and resistors behave very differently in circuits. The resistor’s behavior is linear, according to Ohm’s law
![]() Current-voltage characteristics of typical LED of different colors If the voltage across the resistor is increasing, the current is also proportionally increasing (we assume that the resistor value stays the same). On the other hand, LEDs behave differently. They behave as ordinary diodes according to their current-voltage characteristic curve shown in the picture for LEDs of different colors. The curves show that the current through the LED is not directly proportional to the voltage across it. The current through the LED is dependent exponentially on the forward voltage. That means that only a small change in voltage will cause a large change in current. When the LED forward voltage is small, its resistance is very high. If the voltage reaches the characteristic forward voltage value shown in the specifications, the LED “turns on” and its resistance quickly drops off. If the applied voltage is slightly larger than the LED’s forward voltage, the forward voltage exceeds the recommended value, which can be 1.5 to 4 volts for LEDs of different colors. In this case, the current quickly increases and the diode can be damaged. To limit this current, a resistor is placed in series with the LED to keep the current at a specific level shown in the LED specifications. Calculations![]() A rectangular LED with a flat top used in applications like a bar-graph display The value of the series current limiting resistor Rs can be calculated using Ohm’s law formula in which the supply voltage Vs is offset by the forward voltage drop across the diode Vf:
where Vs is the power supply voltage (for example 5 V USB power) in volts, Vf is the LED forward voltage drop in volts and I is the LED current in amperes. Both Vf and If can be found in the LED manufacturer specifications. Typical values of Vf are shown in the table above. Typical current of LEDs used for indication is 20 mA. After the resistor value is calculated, a nearest higher standard value is selected from the preferred resistor numbers. For example, if our calculation shows that we need a resistor Rs = 145 ohm, we will take a resistor Rsp = 150 ohm. The current-limiting resistor dissipates some power, which is calculated as
![]() Orange LEDs commonly used in routers to show 10/100 Mbps speed; green LEDs show 1000 Mbps speed Usually, the resistor wattage is selected close to twice the value calculated here. For example, if the value of the power is 0.06 W, we will select a resistor with a wattage rating of 0.125 or 1/8 W. Now we will calculate the efficiency, which will show how much of the total power consumed by the circuit is used by the LED. The power dissipated by the LED:
Total power consumption
LED circuit efficiency
To select the power supply, we will calculate the current drawn from the power supply:
![]() A LED strip with 5050 diodes; the numbers 50 and 50 indicate the length and width of the chip in millimeters; the series 150-ohm resistors are pre-installed on the strip LED ArraysA single LED can be driven using a current-limiting resistor. LED arrays that are increasingly used for area illumination, computer monitor and TV display backlighting, and other purposes require specialized power supplies. We are all used to voltage-stabilized power supplies. However, power supplies for LED driving must stabilize their current, not voltage. In any case, current-limiting resistors are always used in LED arrays. If more than one LED is necessary for the application, strings of multiple LEDs connected in series can be used. For a string of LEDs connected in series, the source voltage must be greater than or equal to the sum of voltages across individual LEDs. If it is greater, then a single current-limiting resistor per string can be used. The current through each diode is identical, which ensures uniform brightness. It is generally best if all LEDs connected in series are all of the same type. However, in the case of a single LED open-mode failure, which is the most common failure mode, the entire LED string goes dark. In some designs, a special shunt protection device is used to prevent this. Zener diodes connected in parallel with each LED can be used for this purpose. This approach is good for low-power LEDs, but for high-power LEDs used, for example, in street lighting, this approach is not cost-effective, and more complex shunt protectors must be used. Of course, this adds to cost and space requirements. At present (2018), it can be observed that LED street lights, with a planned service life of 10 years, serve no more than a year. The same applies to household LED lamps, including lamps made by well-known manufacturers. ![]() This LED strip is used for illumination of the LCD television panel; two such strips are installed on both sides of the screen panel. This design allows the thinnest displays. Note that televisions with LED-backlit LCD panels are commonly marketed as LED TVs. The real LED TVs use OLED displays. When calculating the required resistance of the current limiting resistor Rs, all voltage drops across each LED needs to be considered. For example, if a voltage drop across each illuminated LED is 2 V and we connected five LEDs in series, then the total voltage drop across all five will be 5 × 2 = 10 V. Several identical LEDs can be also connected in parallel. Parallel LEDs must have matched forward voltages Vf, otherwise, they will not have a similar current through them and therefore their brightness will be different. For a parallel connection of LEDs, it is a good idea to connect a current-limiting resistor in series with each diode. In the parallel connection, an open-circuit failure of any single diode will not lead to the entire diode set going dark — it will be operated normally. Another problem of the all-parallel connection is the choice of an efficient low-voltage and high-current power supply, which, at the same power rating, can be more expensive than conventional power supplies for higher voltages and lower currents. ![]() In this common LED street lighting fixture 8 strings of 5 powerful LEDs for a total of 40 LEDs are driven by an efficient constant current power supply; note that two strings (top left and bottom right) are dark in this fixture installed only a couple of months ago because in each of them one diode failed and protection devices are not used or not working Calculations of the Current-Limiting ResistorsIf the number of LEDs in a series string NLEDs in string (denoted as Ns in the entry field) is not entered, then it will be determined here. The maximum number of LEDs in a series string NLEDs in string max for a given power supply voltage Vs and LED’s forward voltage Vf:
If the number of LEDs in a series string NLEDs in string (denoted as Ns in the entry field) is entered then the maximum number of LEDs in a series string NLEDs in string max is determined as
![]() A 3014 (3.0 × 1.4 mm) SMD LED used in LCD TV with LED backlight The number of strings with the maximum number of LEDs in the string Nstrings:
The number of LEDs in the remainder shorter string Nremainder LEDs:
If Nremainder LEDs = 0 then there will be no additional string. The resistance of the current limiting resistor for strings with the max. number of LEDs:
The resistance of the current limiting resistor for strings with fewer LEDs than the max. number of LEDs:
The common power PLED dissipating by all light emitting diodes:
The power dissipating by the resistors:
![]() Flexible LED displays in a public place; a LED display uses an array of light-emitting diodes as pixels; because of LED very high brightness, they are commonly used outdoors as billboards or highway destination sights that are visible in bright sunlight. LED displays can also provide general illumination and are often used as photo and video lighting with variable color temperature The power rating is determined with a safety factor k = 2, which ensures the reliable operation of the resistor. Select the resistor power rating, which is twice higher than the calculated power from the following values: 0.125; 0.25; 0.5; 1, 2, 3, 4, 5, 8, 10, 16, 25, 50 W. Calculating the common power PR dissipating by all resistors:
Calculating the common power Ptotal dissipating by the array:
Calculating the current that the array draws from the power supply:
Calculating the array efficiency:
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